Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Fuel cells include three components: a cathode, an anode and an electrolyte, which is sandwiched between the cathode and the anode and ideally passes only protons. Each electrode is coated on one side by with a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out. Moreover, each proton that migrates through the polyelectrolyte membrane has a hydration sphere that in theory consists of at least four water molecules. The resultant electro-osmotic drag results in the drying out of the anode side of the membrane, especially as large numbers of hydrated protons move through the membrane. Because water is formed at the cathode side of the membrane, water molecules must diffuse back from the cathode side of the membrane to the anode side to sustain membrane hydration and to provide channels for vehicular proton transport. Thin membranes more easily remain uniformly hydrated compared with thicker membranes, because the back diffusion of water is more facile in thinner membranes. Moreover, with all else being equal, the electrical resistance of a thin membrane is less than that for of a thicker membrane. Furthermore, improved electrical resistance is also improved due to the ionomer treated membrane's ability to better provide a match to the ionomer in the catalyst layer. The catalyst layer often includes ionomer that is added during the catalyst preparation process to improve activity. Thus, improved electrical resistance in the ionomer treated membrane of the present invention is partly due to reduced dimensional thickness and is partly due to a reduced contact resistance between the ionomer treated membrane and the catalyst layer. The latter is a consequence of having treated the membrane film with an ionomer that better matches the ionomer in the catalyst layer.
In order to prevent leakage of the hydrogen fuel gas and oxygen gas supplied to the electrodes and prevent mixing of the gases, a gas-sealing material and gaskets are arranged on the periphery of the electrodes, with the polymer electrolyte membrane sandwiched there between. The sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane to form a membrane and electrode assembly (MEA). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and electrically connecting adjacent MEAs in series. A portion of the separator plate, which is disposed in contact with the MEA, is provided with a gas passage for supplying hydrogen fuel gas to the electrode surface (on the anode side) and removing generated water vapor (from the cathode side).
During fabrication of a fuel cell, the polymer electrolyte membrane of each MEA is produced in roll form under tension. The polymer electrolyte membrane has a high water uptake capability. Therefore, when wet, the membrane will expand in all three directions, although not proportionally. The membrane will shrink in all three dimensions upon subsequent drying.
Because the proton conductivity of PEM fuel cell membranes deteriorates rapidly as the membranes dry out, external humidification is required to maintain hydration of the membranes and sustain proper fuel cell functioning. Moreover, the presence of liquid water in automotive fuel cells is unavoidable because appreciable quantities of water are generated as a by-product of the electrochemical reactions during fuel cell operation. Furthermore, saturation of the fuel cell membranes with water can result from rapid changes in temperature, relative humidity, and operating and shutdown conditions. However, excessive membrane hydration results in flooding, excessive swelling of the membranes and the formation of differential pressure gradients across the fuel cell stack.
In order to maintain consistent fuel cell stack pressures, membranes are needed which swell uniformly and then only marginally in the presence of liquid water. Perfluorosulfonic acid (PFSA) membranes are typically used because of their advantaged oxidative, chemical and thermal stability and because of their superior proton conductivities at low relative humidity. PFSA membranes with a wide range of physical properties are available, and performance depends on the membrane's ion exchange capacity and the internal stresses and defects introduced during the membrane-film preparation process.
PFSA membranes with high acid numbers (or low equivalent weights) have enhanced proton conductivity at reduced relative humidity, but the mechanical properties of these membranes (especially with the high acid numbers) are compromised because of swelling due to high water uptake. The in-plane swelling of extruded membranes is further complicated because the membranes typically swell less in the machine process direction (the x-axis) as compared to the transverse direction (the y-axis) of the film. Non-uniform membrane swelling introduces the possibility of uneven, pressure-related stress failure mechanisms in fuel cell stacks.
Improved proton conducting membranes are required to meet cost and durability targets for polyelectrolyte membrane (PEM) fuel cells in automotive applications. Presently, PEM fuel cells operate at temperatures of up to about 95 degrees C., and external humidification is required to maintain membrane hydration. As the membranes dry out at reduced humidity, proton conductivity deteriorates rapidly. When fuel cells operate near the boiling point of water, elevated pressures are required to maintain membrane hydration, and the compressors required are a parasitic drain on the energy produced. PSFA membranes with high acid numbers (or low equivalent weights) are expected to have enhanced proton conductivity at reduced relative humidity, but the mechanical properties of these membranes (with the high acid numbers) are compromised because of increased water uptake and swelling. Cross-linking has been proposed to prevent the physical degradation of membranes with high acid numbers. However, methods are needed to perform the cross-linking reactions after the film is made; otherwise, film manufacture is difficult. The corporation W.L. Gore markets composite membranes made with low equivalent weight, PFSA ionomer. The structural strength of the membrane is maintained by reinforcing the ionomer with a porous expanded polytetrafluoroethylene matrix. Alternatively, a technique has been found to improve the performance of PFSA membranes with low acid numbers by imbibing, for example but not limited to, low equivalent weight ionomer into preformed high equivalent weight PFSA films. Moreover, the immersion (treatment) of a perfluorosulfonic acid film or other polyelectrolyte film with a dispersion (or solution) of perfluorosulfonic polymer causes the film to swell, and after removal and drying under tension, the modified membrane then becomes thinner (by about one-half) than that of the untreated film, and internal film stresses are reduced in the membrane film. The membrane film is found to have reduced electrical resistance compared with that of the untreated film. This improved electrical resistance is in part due to the thinner membrane and in part due to a better match of the catalyst layer (which often includes polyelectrolyte added during catalyst preparation) and the membrane.